Surround Sound vs Dolby Atmos vs Spatial Audio

[struts]

Some further digging confirms that Dione is not compatible with Apple's Spatial Audio (list of compatible devices here).  Makes sense per my post above.

Also, it seems that Dione cannot currently be configured as a "home cinema audio" speaker with Airplay 2, which is a prerequisite for supporting the playback of Dolby Atmos content over Airplay (see here).

Here's what you need

• Apple TV 4K
  
• One or two HomePod mini* or HomePod speakers. Other AirPlay 2-compatible speakers are not supported with home cinema audio.
  

* HomePod mini doesn't support Dolby Atmos 5.1 or 7.1 surround sound.

It is not quite clear to me whether this is a technology restriction or commercial lock-out by Apple.  What is clear is that the ability to configure HomePods as home cinema speakers is based on support for HomeKit as well as Airplay 2 and Dione doesn't (yet?) support HomeKit.

So, bottom line as I read it:

• Dione does not support Spatial Audio.
  
• Dione can render Dolby Atmos content from an Apple TV 4K when the two are connected via HDMI but not via Airplay 2.
  
• Dione cannot be combined with Phantoms, HomePods, or any other speakers in a Dolby Atmos surround sound configuration.  
  
• The only speakers that can currently play Dolby Atmos content via Airplay 2 are one or two HomePod mini or HomePod speakers (in which case the Atmos content is presumably downmixed to 1.0.0 or 2.0.0 thereby losing most of its “spatialness”, although I could find no explanation of this on Apple's website)
  
• It is possible to play content simultaneously via Dione and Phantoms or HomePods using Airplay 2's multi-speaker support but in that case the Phantoms/HomePods and the Dione would receive identical stereo signals so it wouldn't be surround sound.
  

Surround sound really does seem to be a can of worms!   Once again, delighted if anyone can shed further light or put me straight if any of this is incorrect.